1. Field of the Invention
This present invention relates to an improved algorithm for smoothing artificial discontinuities between image blocks (blocking artifacts) and reducing ringing noise, without introducing undesired blur. The invention, which can be embodied in an apparatus, methods or programs of instructions, is particularly, though not exclusively, designed for low-bit-rate video coding applications.
2. Description of the Related Art
Today's low-bit-rate video coding standards, MPEG-4 and ITU-T H.263, contain algorithms that enable a variety of applications, including video conferencing and wireless video communications. These standards and the systems that use them take advantage of temporal redundancy as well as spatial redundancy to compress video. While these standards are quite effective in many ways, they sometimes generate decompressed images that exhibit artificial discontinuities between image blocks. These blocking artifacts are caused primarily by quantization during the quantization step of the compression process.
To combat this problem these coding standards have incorporated algorithms for reducing blocking artifacts. In block-based coding, monotone areas of the original image, where the pixel intensity changes gradually, suffer most noticeably from the abrupt changes across the block boundary, leading to blocking artifacts. In terms of discrete cosine transform (DCT), when the DCT coefficient quantization step size is above the threshold for visibility, discontinuities in grayscale values are caused by removal of AC coefficients due to quantization and become clearly visible at the boundaries between blocks. Also, visually annoying noise near real edges, which is called ringing noise, is inevitable in the block-based DCT scheme. In applications using low bit-rate coded video sources, such blocking and ringing artifacts cause substantial practical problems, and therefore must be reduced. Various deblocking schemes have been proposed in still image coding such as JPEG under the assumption that blocking artifacts are always located at block boundaries. A well-known method for reducing blocking artifacts is based on the theory of alternative projection onto convex sets (POCS). However, this method is only applied to still images because of an iteration structure and long convergence time.
In video coding, unlike still image coding, blocking artifacts might occur inside a block since the blocking artifacts of the previous frame can be propagated to the current frame. Recently, post-filtering methods for low bit-rate coded video have been proposed. For example, an adaptive filtering scheme has been proposed based on local characteristics to reduce blocking artifacts. In another proposal a frequency-domain analysis is conducted for extraction of the block semaphores from the intracoded picture. The semaphores extracted are propagated using inverse motion compensation for inter-frame de-blocking and de-ringing. While these techniques are generally acceptable, further improvements, especially in the areas of increased speed and reduced computational complexity, are desirable.